Various methods have been known as the methods for producing seamless steel pipes. For example, the Mannesmann-mandrel mill method is a method in which a heated billet is subjected to piercing-rolling with a piercing mill, an elongation rolling with a mandrel mill, and further to a sizing with a sizing mill or the like.
The applicant has disclosed in Patent Document 1, with respect to the method for determining the dimension of a billet, an invention of “a method for producing a seamless steel pipe which method is provided with a rolling mill capable of regulating the dimension of a product and in which method a seamless steel pipe is produced in a production line with a cutting machine disposed downstream of a heating furnace, wherein: even when the products to be produced are different in dimension from each other, the stocks for the products are collected into groups so as for each of the groups to consist of the same type of stocks; in each of the groups, the stocks each ensure the stock length that is equal to or longer than the minimum length constrained by the stock type; and in each of the groups, the thus qualified stocks are conveyed and heated and then cut to required lengths with a cutting machine, and then the cut stocks are rolled so as to have the predetermined product dimensions.”
Patent Document 1: JP2001-105012A